Biodiesel fuels (BDF (registered trademark)) made of long chain fatty acid alkyl esters are obtained using vegetable oil/fats or animal oil/fats, which are natural products, as raw materials, and have therefore been introduced and become widely used in Europe and Southeast Asia as environmentally friendly alternatives to diesel fuel. European standard EN14214, Japanese Industrial Standard JIS K2390 and American National Standard ASTM D6751 have been established as specifications that stipulate product quality for the safe use of biodiesel fuels as vehicle fuels, and these quality standards must be satisfied when biodiesel fuels are mixed with diesel oil and supplied to the market.
As the use of biodiesel fuels has progressed, a variety of new problems have become apparent when biodiesel fuels are used in vehicles. Among these, a major problem that has been reported is that components that are readily precipitated in biodiesel fuels precipitate in fuel storage tanks and in filters in engine systems, which leads to problems such as clogging. Fatty acid monoglycerides (and especially saturated fatty acid monoglycerides) and steryl glycosides are given as examples of components that precipitate. As a result, specifications for reducing problems when biodiesel fuels are used in automobiles have been reviewed, and there has been a tendency for regulation values for fatty acid monoglycerides to become more stringent. In European standard EN14214, for example, the permitted value for fatty acid monoglycerides was 0.8 mass % or lower when the standard was established in 2003, but this upper limit was lowered to 0.7 mass % or lower when the standard was revised in 2013, and a further reduction has been demanded.
Fatty acid monoglycerides are an intermediate product in reactions in which biodiesel fuels are produced when transesterification reactions are carried out with oils and fats, and because transesterification reactions are equilibrium reactions, fatty acid monoglycerides are difficult to remove by means of reaction alone. As a result, attempts have been made to remove fatty acid monoglycerides and other glycerides by subjecting obtained biodiesel fuels to a variety of purification processes. Such biodiesel fuel purification processes include washing with water, distillation, removal by means of adsorbents, extraction and membrane separation. FIG. 1 is a diagram showing the outline of biodiesel fuel purification processes by means of conventional methods.
Fatty acid monoglycerides have 2 free hydroxyl groups in the glycerin moiety of the molecular structure, and are therefore slightly water-soluble. Attempts have been made to remove fatty acid monoglycerides by means of water washing by utilizing this water solubility. In removal methods involving washing with water, purified biodiesel fuel contains moisture, meaning that the biodiesel fuel does not meet standards in this state. As a result, a water-oil separation filter is used in order to remove the moisture (see PTL 1). In addition, water washing that also involves the use of a flocculant is carried out in order to prevent loss of methyl esters of fatty acids, which are primary components of biodiesel fuels obtained by means of water washing (see PTL 2).
Purification by distillation is carried out as a method for removing fatty acid monoglycerides (see PTL 3 to PTL 6). Such methods are effective for removing moisture and removing diglycerides and triglycerides, which have far higher boiling points than methyl esters of fatty acids, but fatty acid monoglycerides have similar boiling points to methyl esters of fatty acid, meaning that separation performance of fatty acid monoglycerides is insufficient when carrying out purification by distillation.
Removal and purification by using an adsorbent is carried out as a method for removing fatty acid monoglycerides (see PTL 7 to PTL 16). Silica gel, clay minerals (activated clay, montmorillonite, and the like), zeolites, diatomaceous earth, magnesium silicates, alumina, and the like, are used as adsorbents. Methods involving the use of an adsorbent achieve higher separation performance than washing with water or distillation, but because there is a limit to the amount adsorbed, a large quantity of adsorbent is required in cases where the quantity of fatty acid monoglycerides is high, meaning that adsorption performance depends on the type of adsorbent and the composition of the raw material oil (the crude biodiesel fuel) and spent adsorbent needs to be regenerated or disposed of.
Other methods for removing fatty acid monoglycerides include liquid-liquid extraction methods involving the use of a solvent (see PTL 17) and removal methods involving the use of a separation membrane (see PTL 18), but separation efficiency is not high and a complex procedure is required.